Byung-Koog Jang

Thermophysical properties of porous SiC ceramics fabricated by pressureless sintering

Abstract Highly porous SiC with approximately 30–41% porosity was fabricated by pressureless sintering without sintering additives at temperatures in the range 1700–2000 °C. Thermal diffusivities, specific heats, thermal conductivities and thermal resistivities of sintered samples are reported for temperatures from room temperature to 1000 °C. The thermal diffusivities and thermal conductivities of all samples decreased significantly with increasing temperature over this range, whereas specific heats and thermal resistivities increased. At any given temperature, the greater the porosity of the SiC, the lower the thermal conductivity.

Evaluation of thermal conductivity of zirconia coating layers deposited by EB-PVD

Electron beam-physical vapor deposition (EB-PVD) is a widely used technique for depositing thermal barrier coatings (TBCs) on metal substrates for high temperature applications, such as gas turbines, in order to improve thermal efficiency [1]. Characterization of the thermal conductivity of the coating layers is therefore very important for developing superior thermal barrier coatings, but because of the irregular nature of the coated specimens it is difficult to derive the thermal conductivity of the coating layer from measurements of the thermal conductivity of the combined coating and substrate. Two steps are therefore involved in determining the thermal conductivity of a thin coating film: (i) separation of the coating film from the combined coating and substrate specimen, and (ii) measurement of the thermal conductivity of the film. With regards to the first step, it is known that coating layers deposited by EB-PVD have a porous structure so that they are easily damaged because of their poor strength [2, 3]. In other words, in practice it is not easy to physically separate the coating film from the coated substrate by machining or some other method without damaging it. Regarding the second step, even if the coating can be successfully separated from the substrate, it is not a simple matter to measure directly the thermal conductivity of the coating. The laser flash method is generally used to accurately measure the thermal diffusivity and specific heat capacity of materials, from which the thermal conductivity can be calculated. The technique was developed by Parker et al. [4], and is usually carried out assuming the specimen to be uniformly dense and opaque. However, coated layers deposited by EB-PVD have a columnar non-uniform structure, making it difficult to measure the thermal conductivity directly. The aim of the present work is therefore to derive a practical method for determining the thermal conductivity of coating layers based on theoretical calculations, and comparing the values obtained with direct experimental measurements. We have therefore adopted the response function method as a means of determining the thermal conductivity of the coating layers. It has been reported that the response function method is a powerful method to analyze one-dimensional heat diffusion across multi-layer materials [5]. We also present the experimental results from thermal conductivity measurements of coated substrates as well as coating layers detached from their substrates as a function of substrate thickness. In this work, ZrO2-4 mol% Y2O3 coatings have been applied by EB-PVD to zirconia substrates with the same composition as the coating material to minimize interface effects on thermal conductivity. Disc-type zirconia substrates were prepared by pressureless sintering at 1600◦C. The sintered substrates were machined to 10.0 mm diameter and 0.1–3 mm thickness. The substrates were first preheated at 900–1000 ◦C in a heating chamber using a graphite heating element. An electron beam evaporation process was used to deposit the film in a coating chamber under a vacuum level of 10−4 Pa using a 45 kW electron gun at a rate of 4 μm/min and substrate rotation speed of 5 rpm. The average coating thickness was about 300 μm. The density of each specimen was determined by measuring its mass on an electronic balance and its volume with a micrometer. All thermal diffusivity and specific heat capacity measurements were carried out three times for each specimen at room temperature by the laser flash method. The microstructure of the coated specimens was observed by SEM. A typical microstructure of a specimen coated on a zirconia substrate is shown in Fig. 1. The crosssectional surface of the coated specimen clearly reveals the columnar microstructure, with all columnar grains oriented in the same direction, i.e., perpendicular to the substrate. This columnar structure is very similar to those reported for metal substrates coated by EB-PVD [3]. In other words, the distinctive columnar microstructure can be obtained regardless of whether the substrate being used is metal or ceramic. The correlation between temperature rise at the rear surface of a specimen and time is shown in Fig. 2 when the front surface of the specimen is uniformly heated using a laser pulse. For bulk materials, the thermal diffusivity (α) is described by the following equation:

Alignment of carbon nanotubes by magnetic fields and aqueous dispersion

Homogeneous dispersion of multi-walled carbon nanotubes (CNTs) in aqueous solution was achieved using several dispersants. The most efficacious dispersant was Polyethyleneimine (PEI). Stable CNT dispersions are found to have higher zeta potentials compared to poorly dispersed suspensions. Application of a strong magnetic field of 12 T to CNT to align in the direction of the magnetic field. Well-aligned CNTs according to direction of magnetic field were obtained.

Characterization of multiwalled carbon nanotube-reinforced hydroxyapatite composites consolidated by spark plasma sintering.

Pure HA and 1, 3, 5, and 10u2009vol% multiwalled carbon nanotube- (MWNT-) reinforced hydroxyapatite (HA) were consolidated using a spark plasma sintering (SPS) technique. The relative density of pure HA increased with increasing sintering temperature, but that of the MWNT/HA composite reached almost full density at 900°C, and then decreased with further increases in sintering temperature. The relative density of the MWNT/HA composites increased with increasing MWNT content due to the excellent thermal conductivity of MWNTs. The grain size of MWNT/HA composites decreased with increasing MWNT content and increased with increasing sintering temperature. Pull-out toughening of the MWNTs of the MWNT/HA composites was observed in the fractured surface, which can be used to predict the improvement of the mechanical properties. On the other hand, the existence of undispersed or agglomerate MWNTs in the MWNT/HA composites accompanied large pores. The formation of large pores increased with increasing sintering temperature and MWNT content. The addition of MWNT in HA increased the hardness and fracture toughness by approximately 3~4 times, despite the presence of large pores produced by un-dispersed MWNTs. This provides strong evidence as to why the MWNTs are good candidates as reinforcements for strengthening the ceramic matrix. The MWNT/HA composites did not decompose during SPS sintering. The MWNT-reinforced HA composites were non-toxic and showed a good cell affinity and morphology in vitro for 1 day.

Structure and Thermal Conductivity of Thermal Barrier Coatings in Lanthanum/Gadolinium Zirconate System Fabricated via Suspension Plasma Spray

High Temperature Materials Unit, National Institute for Materials Science, Tsukuba, 305-0047, Japan(Received December 3, 2014 ; revised December 24, 2014 ; accepted December 24, 2014)AbstractWith increase in demand for higher operating temperatures of gas turbines, extensive research efforts havebeen carried out to enhance the performance of thermal barrier coatings (TBCs) in the field of coating pro-cessing as well as materials. In this study, thermal barrier coatings in lanthanum/gadolinium zirconate system,which is one of the most promising candidates for replacing yttira-stabilized zirconia (YSZ) in thermal barriercoating applications, are fabricated via suspension plasma spray. Dense, 300 ~ 400 µm thick coatings of fluorite-phase zirconate with modest amount of segmented microstructures are obtained by using suspension plasmaspray with suspensions of planetary-milled mixture between lanthanum and/or gadolinium oxide and nanozirconia. These coatings exhibit thermal conductivities of 1.6 ~ 1.7 W/mK at 1000

Fabrication of Dense ZrO2/CNT Composites: Influence of Bead-Milling Treatment

Highly concentrated zirconia-carbon nanotube (CNT) water suspensions were prepared using an advanced milling technique. The bead-milling operation parameters were optimized for this system and used to prepare zirconia-stabilized water-based suspensions with different CNT contents. The effects of different milling conditions were studied. The particle dispersion was evaluated by SEM observations on dried suspension. Green’s density and SEM observations of compacts were used to follow the colloidal dispersability of the composites. Materials of tetragonal zirconia and CNTs were prepared with a high concentration of CNTs (1, 5, and 10xa0wt pct CNT). The homogeneous dispersion and distribution of the fibers in the bulk material after slip casting of the suspension were examined. The samples were sintered using spark plasma sintering (SPS) at 1473xa0K (1200xa0°C) and finally, fully dense materials were obtained. The mechanical properties were evaluated using the Vickers indentation technique.

Fiber reinforced composites as self-diagnosis materials for memorizing damage histories

Electrically conductive fiber-reinforced composites have been designed in order to develop self-diagnosis materials with the ability to memorize damage histories. Irreversible resistance changes dependent on the strain histories of the composites were utilized to achieve this ability. Conductive fiber-reinforced plastics for memorizing maximum strain were prepared by adding carbon fibers or particles into the composites. Pre-tensile stresses in composites containing carbon fibers were found to effectively enhance their residual resistance and to significantly improve the limit of smallest detectable strains. The residual resistances of composites containing carbon particles connected by a percolation structure were found to depend strongly on the volume fractions of carbon particles; composites with high volume fractions of carbon displayed remarkable residual resistance without application of a pre-tensile stress. In order to memorize cumulative damage, composites consisting of a brittle titanium nitride ceramic wire laminated with glass fiber reinforced plastics were prepared. These composites were found to exhibit remarkable residual resistances that increased in proportion to the logarithm of the number of tensile cycles. These results suggest that a simple and low cost monitoring technique without real-time measurement system will be available in wide range of applications using these composites.

Preparation of Suspension in La 2 O 3 -Gd 2 O 3 -ZrO 2 System via Planetary Mill and Characteristics of (La 1-x Gd x ) 2 Zr 2 O 7 Coatings Fabricated via Suspension Plasma Spray

Lanthanum/gadolinium zirconate coatings are deposited via suspension plasma spray with suspensions fabricated by a planetary mill and compared with hot-pressed samples via solid-state reaction. With increase in processing time of the planetary mill, the mean size and BET surface area change rapidly in the case of lanthanum oxide powder. By using suspensions of planetary-milled mixture between lanthanum or gadolinium oxide and nano zirconia, dense thick coatings with fully-developed pyrochlore phases are obtained. The possibilities of these SPS-prepared coatings for TBC application are also discussed.

Phase Evolution and Thermal Conductivities of (La 1-x Gd x ) 2 Zr 2 O 7 Oxides for Thermal Barrier Coatings

With increase in operating temperature of gas turbine for higher efficiency, it is necessary to find new materials of TBC for replacement of YSZ. Among candidate materials for future TBCs, zirconate-based oxides with pyrochlore and fluorite are prevailing ones. In this study, phase structure and thermal conductivities of oxide system are investigated. system are comprised by selecting as A-site ions and as B-site ion in pyrochlore structures. With powder mixture from each oxide, oxides are fabricated via solid-state reaction at . Either pyrochlore or fluorite or mixture of both appears after heat treatment. For the developed phases along compositions, thermal conductivities are examined, with which the potential of compositions for TBC application is also discussed.

Preparation of YSZ Electrolyte for SOFC by Electron Beam PVD

Yttria stabilized zirconia (YSZ) films with the thickness of up to 12 μm were prepared on alumina and NiO-YSZ substrates by electron beam physical vapor deposition (EB-PVD). The films showed nano-scaled columnar structures depending on the substrate temperature. Electrical conductivity of the YSZ films on alumina was also investigated at the temperature between 700 and 1000oC in oxidizing atmosphere. High activation energy of the conductivity (>1.03eV) indicated that the conduction via grain boundary controlled the ionic conduction in the films prepared by EB-PVD. La0.6Sr0.4CoO3-δ as a cathode was applied on the YSZ/NiO-YSZ in order to evaluate the performance of the YSZ electrolyte.